CSE443 Compilers Dr. Carl Alphonce alphonce@buffalo.edu 343 Davis - PowerPoint PPT Presentation

CSE443 Compilers Dr. Carl Alphonce alphonce@buffalo.edu 343 Davis Hall

Phases of a compiler Target machine code generation Figure 1.6, page 5 of text

7.2.3 Calling Sequence What happens during a function call?

actual parameters caller's returned value invocation control link record access link saved machine status top_sp local data temporaries top Prior to function call.

7.2.3 Calling Sequence "Procedure calls are implemented by what are known as calling sequences, which consist of code that allocates an activation record on the stack and enters information into its fields." [p. 436]

actual parameters caller's returned value invocation control link record access link saved machine status local data temporaries actual parameters During function call. callee's returned value invocation control link record access link saved machine status top_sp local data temporaries top

7.2.3 Calling Sequence " A return sequence is similar code to restore the state of the machine so the calling procedure can continue its execution after the call." [p. 436]

actual parameters caller's returned value invocation control link record access link saved machine status top_sp local data temporaries top … After function call. returned value … … … … …

Caller vs Callee responsibilities "In general, if a procedure is called from n different points, then the portion of the calling sequence assigned to the caller is generated n times. However, the portion assigned to the callee is generated only once." [p. 436]

Typical calling sequence [p. 437] "1. The caller evaluates the actual parameters." Recall: formal parameter == parameter actual parameter == argument

actual parameters caller's returned value invocation control link record access link saved machine status top_sp local data temporaries top Prior to function call.

actual parameters caller's returned value invocation control link record access link saved machine status top_sp local data temporaries top Caller writes actual parameters arguments (actual parameters) past the end of its own invocation record.

actual parameters caller's returned value invocation control link record access link saved machine status top_sp local data Caller knows the temporaries top offset of the eventual actual parameters returned value. When callee returns the returned value caller will look at this location for the returned value.

actual parameters caller's returned value invocation control link record access link saved machine status top_sp local data temporaries top actual parameters The caller stores its stack pointer here. returned value control link

Typical calling sequence [p. 437] "2. The caller stores a return address and the old value of top_sp into the callee's activation record. … "

actual parameters caller's returned value invocation control link record access link saved machine status top_sp The caller stores local data its stack pointer here. When the temporaries callee finishes the top stack pointer's value actual parameters will be reset to this value, thereby returned value restoring the caller's top_sp invocation record as the active one (the one on top of the stack).

Typical calling sequence [p. 437] "The caller then increments top_sp […] top_sp is moved past the caller's local data and temporaries and the callee's parameters and status fields."

actual parameters caller's returned value invocation control link record access link saved machine status local data temporaries actual parameters Move top_sp returned value top_sp access link saved machine status top_sp … …

Typical calling sequence [p. 437] "3. The callee saves the register values and other status information."

actual parameters caller's returned value invocation control link record access link saved machine status Write the return local data address, the current value of the Program temporaries top Counter (PC), into the actual parameters saved machine status. When the callee returned value finishes execution will resume with the top_sp address pointed to by this saved access link address. PC + machine status top_sp … …

actual parameters caller's returned value invocation control link record access link saved machine status local data When control transfers to the temporaries callee, the top_sp and actual parameters top are updated. callee's returned value Callee writes local invocation data and temporaries top_sp into its invocation record record. access link PC + machine status top_sp local data temporaries top

actual parameters caller's returned value invocation control link record access link saved machine status If the number of local data arguments can vary from call to call temporaries (e.g. printf) then the caller writes the actual parameters arguments to the callee's "actual parameters" 
 returned value area, as well as invocation top_sp information about the number of record access link arguments to the status area PC + machine status top_sp local data temporaries top

actual parameters caller's returned value invocation control link record access link saved machine status If the callee has variable local data length local data (e.g. local arrays temporaries whose size is determined by the actual parameters value of a parameter) callee's then the arrays are returned value allocated space at the invocation top_sp end of the invocation record, and pointers record access link to those arrays are stored in the PC + machine status "locals" block. top_sp local data temporaries top

Relocatable object code Compiler produces relocatable object code: addresses are not absolute, but relative to known boundaries (e.g. Stack Pointer, start of record, Program Counter). Linker combines object code files into an executable file, in which static relative addresses are made absolute (in virtual address space). Loader copies contents of executable file into memory and starts execution.

Relocatable object code Compiler produces relocatable object code: addresses are not absolute, but relative to known boundaries (e.g. Stack Pointer, start of record, Program Counter). Linker combines object code files into an executable file, in which static relative addresses are made absolute (in virtual address space). Leave relative offsets alone during Loader copies contents of executable file into translation. memory and starts execution.

Desirable characteristics of generated code: correctness (this is non-negotiable) small execution time small code size small power consumption

Desirable characteristics Associate costs with each instruction, of generated code: then "minimize" (lower) overall cost, with some balance since execution time and code size can correctness (this is non-negotiable) be in conflict. small execution time small code size small power consumption

Significant tasks of code generator instruction selection register allocation and assignment instruction ordering

Significant tasks of code generator Which variables are kept in registers? instruction selection register allocation and assignment instruction ordering

Significant tasks of code generator Which specific register holds which value? instruction selection register allocation and assignment instruction ordering

Significant tasks of code generator E.g. to minimize the number of registers needed. instruction selection register allocation and assignment instruction ordering

Simple generation strategy vs. code size If we generate code for each intermediate code instruction in isolation and string the results together the result may include redundant instructions

Small example [p. 509] Consider: x = y + z This might be translated as: LD R0, y <— load the value of y into register R0 ADD R0, R0, z <— put into R0 the result of adding R0 and the value of z ST x, R0 <— store the value of register R0 to x

Larger example [p. 509] Consider applying the same template to a larger example: a = b + c d = a + e This might be translated as: LD R0, b ADD R0, R0, c ST a, R0 LD R0, a ADD R0, R0, e ST d, R0

Larger example [p. 509] Consider applying the same template to a larger example: a = b + c d = a + e This might be translated as: LD R0, b ADD R0, R0, c ST a, R0 This instruction is LD R0, a redundant: it is loading into ADD R0, R0, e R0 the value that is already ST d, R0 there.